Adjustable stable power supply for continuous wave magnetron

ABSTRACT

A power supply circuit for a continuous wave magnetron provides adjustable stable power levels. A first direct voltage source is connected across the magnetron and a second direct voltage source from a common alternating voltage source is connected across the magnetron coil in series with a direct current amplifier. Current feedback to the input of the amplifier is derived from the magnetron by an adjustable voltage divider connection to the first voltage source. Another portion of the voltage divider provides a third direct voltage connection between the second source and amplifier input, the amplifier input current being the sum of current from the third source and adjustable current from the magnetron.

United States Patent Poussereau et al.

[ 51 Mar. 27, 1973 ADJUSTABLE STABLE POWER SUPPLY FOR CONTINUOUS WAVEMAGNETRON Inventors: Maurice Francois Poussereau, Conflans SaintHonorine; Marcel Denis, Paris, both of France Assignee: InternationalStandard Electric Corporation, New York, N.Y.

Filed: Mar. 30, 1972 Appl. No.: 239,529

Foreign Application Priority Data Apr. 7, 1971 France ..71 12281 US. Cl...315/39.51, 328/230, 330/47, 331/86 Int. Cl. ..H0lj 25/50 Field ofSearch ......3l5/39.51; 328/230; 331/86; I 330/47 [56] References CitedUNITED STATES PATENTS 3,302,060 l/l967 Bluk et al. ..331/86 3,344,3649/1967 Alford ..33l/86 Primary Examiner-- John K'ominski AttorneyC.Cornell Remsen, Jr. et al.

ABSTRACT A power supply circuit for a continuous wave magnetron providesadjustable stable power levels. A first direct voltage source isconnected across the magnetron and a second direct voltage source from acommon alternating voltage source is connected across the magnetron coilin series with a direct current amplifier. Current feedback to the inputof the amplifier is derived from the magnetron by an adjustable voltagedivider connection to the first voltage source. Another portion of thevoltage divider provides a third direct voltage connection between thesecond source and amplifier input, the amplifier input current being thesum of current from the third source and adjustable current from themagnetron.

6 Claims, 2 Drawing Figures ADJUSTABLE STABLE POWER SUPPLY FORCONTINUOUS WAVE MAGNETRON BACKGROUND OF THE INVENTION 1. Field of theInvention This invention relates to improvements in power supplies forcontinuous-wave magnetrons and particularly to a device providing anadjustable stable magnetron power supply.

2. Description of the Prior Art In a previous US. Pat. application Ser.No. 179,384, filed Sept. 10, 1971 and assigned to the same assignee asthe instant application, there is described a power supply for acontinuous-wave magnetron wherein the control induction field isproduced by means of a coil through which passes the sum I of magnetroncurrent intensity I and of current intensity 1,, produced by anadditional source of voltage V, supplying a series resistor R and thecoil resistor R Magnetron high voltage V remaining constant, appliedpower P V I may be adjusted continuously or by steps by varying V and/orR.

The microwave power supplied to the load is equal to P,,, with: P kPkVI; where k is the magnetron efficiency, and from experience it appearsthat, if the load is rather well matched, k is substantially constantthroughout the useful range of the tube characteristics.

Generally, the magnetron characteristics may, in the normal utilizationrange, be represented by the linear relation (1) I=(V/r)aI,, (1) where ris the internal magnetron resistance and a is an amplificationcoefficient of high value. In other words the sensitivity I/I is equalto a and adjustments are unstable when the supply is directly applied tothe coil by providing the current 1,, instead of generating I, by addingI and I, as proposed in the above mentioned prior application.

With the combined supply for the coil, the characteristics arerepresented by the linear relation:

(2) I= (V/aR I and here sensitivity is written: [/1 1 if V, and R areused (2) becomes l B/ )l-( a/ The produced microwave power is A simplecalculation shows that, if V and V are obtained by rectifying a singleAC voltage source U, the relative microwave power variation d"m/P,, isequal to 2 dU/U, for each adjustment of the power P,,,.

An important purpose of the invention described in the above priorapplication is to obtain a satisfactory stabilization of P,, by suitablyamplifying instabilities of V so as to compensate for instabilities ofV.

To do so, the additional source of voltage V is replaced by a source ofvoltage V, that supplies a nonlinear circuit comprising serially acounter electromotive force (c.e.m.f.) element E,,, that is very stableand has a very low internal resistance, the resistor R and the coil ofresistance R,,. V,,is given by the relation:

E is easily obtained by using a Zener diode.

Still according to the above application, a stabilization effect isobtained for a fixed high-voltage V when, for each value of I, andtherefore of V,,' and/or R, E,,, R and I are associated by the relation(5 E, 2 RI 5 By suitably associating a rheostat providing R with avariable autotransformer supplying V it is possible to obtain thepreceding relation for each desired value of P,, and in a continuousmanner.

Under normal conditions of operation for a power supply of the abovetype, there are two drawbacks that have to be overcome. First, itappears that the current of intensity through the coil heats it and, fora relatively long time, the coil resistance increases to stabilize at aheated coil value higher by 20-30 percent than the cold coil value. As aresult, as indicated by the formula (4), there is a slow drift for P,,,,that is sometimes troublesome; the lower the value of R, the higher thevalue of P,,,, and the greater the power drift.

Second, there is a drawback due to transient phenomena appearing whenturning the apparatus on. At that time, it is necessary to be certainthat the volt.- age V, has a value such that the current 1,, through thecoil is higher than the current (1 that blocks the magnetron. In thatcase, it is possible to apply the high-voltage V and to manually reduceV until the occurrence of the desired current I.

In a normal installation, such precautions represent constraints whichusers tend to neglect. But, if when applying high-voltage V which isassumed to occur instantaneously, voltage V, is not sufficient andprovides a current 1,, substantially lower than coil current (1 thatblocks the magnetron, there results a transient mode with rapid increaseof current I. Since the coil self-inductance is not negligible, thereoccurs at the coil terminals a c.e.m.f. L(dl/dt) that is subtracted fromthe auxiliary voltage V, and results in a reduction of the auxiliarycurrent 1,, which may change its direction negatively, and thus furtherincrease I. The phenomenon is unstable and the increase of I is limitedonly by safety switches or non-linear characteristics of the involvedcomponents such as the self-inductance L, Zener diode, magnetron, andrectifiers delivering voltage V The current I increases with atime-constant L/ R. This troublesome phenomenon could be avoided only ifthe increase of current I, after having applied high-voltage V, would beperformed with a time-constant at least equal to L/R.

In any case, such exponentially increasing current is the most damagingto the equipment or, in less unfavorable cases, makes it difficult-tooperate.

SUMMARY OF THE INVENTION Therefore, an object of this invention is toprovide an additional coil current source of adjustable current suchthat the value I B is substantially independent of the coilcharacteristics R and L.

Another object ofthe invention is to provide an additional source insuch a manner that, when high-voltage power is turned on, no transitoryunstable phenomenon occurs.

A further object of this invention is to provide an adjustable powersupply wherein the sensitivity I/I is substantially lower than a.

A still further object of this invention is to provide an additionalcoil current source in such a manner that, for each adjustment of thecoil current I,,, the magnetron current I and the delivered microwavepower P,, are not dependent upon instabilities of the magnetronhigh-voltage V and on an additional source of voltage.

According to a feature of this invention, coil current intensity 1,, isprovided at the output of a current amplifier, the dynamic outputresistance of which is substantially higher than the coil resistor RAccording to another feature of this invention, amplifier input currentis equal to the sum of an adjustable control current I, and of a part qof magnetron current I, I, being lower than coil current I so that whenhighvoltage V is applied to the magnetron, it induces current I. Fromthat feature and from the above mentioned formula (1), it is seen thatmagnetron current is given by the formula:

I=[V/r(l +aq)]-(a/l +aq) 1, Thus, the device sensitivity may be writtenas:

dI/d(I (all +a q) ticularly, the value I, as in the above mentionedapplication. However, excessive transient starting currents are nowavoided due to feedback action proportional to ql, which occurs withoutsubstantial delay.

, According to another feature of this invention, input current 1,, isprovided by a low voltage source V, serially mounted with a negativefeedback voltage equal to r r being a low value resistor. In suchconditions, magnetron current I, at constant high-voltage, andconsequently microwave power P,,,, may be adjusted by varying V,, and/orr,. The sensitivity I/(l,,) is equal to (r,. r;/ r,), r and r being theamplifier input resistor and the sum of internal resistance oflow-voltage source V, and of other resistors possibly serially connectedwith r respectively.

According to another feature of this invention, a stable c.e.m.f. sourcec, with very low internal resistance is serially connected with sourceof low voltage V,,', such that V, V, e e being related to r, and l bythe formula: e, 2 r,l. In such conditions, V being constant, microwavepower P,, is adjusted between desired limits by varying r, and the powervalue is given by the very simple formula:

When the relation e, 2 r 1 is fulfilled and when magnetron supplyhigh-voltage V, low-voltage V, of amplifier output DC supply andlow-voltage V of amplifier input DC supply are provided from a rectifiedsingle AC primary source, power P,, remains stable for any adjustment.The major advantage of such an arrangement is to provide an excellentcontrol of delivered microwave power by using only a non-stabilizedhigh-voltage source and two low-voltage sources, which is much lesscostly than power supplies used in the prior art.

According to another feature of this invention, the amplifieris acommon-emitter power transistor. In such conditions, it is possible touse only one low-voltage source V with V, being obtained from V throughan adjustable potentiometer divider.

Other purposes and features of the present invention will appear moreclearly from the following description in conjunction with theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 shows a schematic diagram of apower supply circuit. for a continuous-wave magnetron according to thisinvention, and

FIG. 2 shows a specific embodiment of a power supply'using a powertransistor as an amplifier.

DESCRIPTION OF THE PREFERRED EMBODIMENTS As shown in FIG. I, a magnetronl is provided with DC high-voltage V from source 2 which is assumed tobe made of a first AC current rectifier of a standard type and of filtercircuits, not shown. The anode of the magnetron l is connected toground. Control induction field for the magnetron is provided by coil 3through which current I is passing; 3 has a resistor value equal to R3.

The bottom terminal of 3 is connected to a positive terminal of a source4 of DC low-voltage V,,' which is assumed to be made of a second ACcurrent rectifier and filter circuits, not shown. The top terminal of 3is connected to the output of a DC amplifier 5 having an outputresistance equal to R,.

Input circuit of amplifier 5 comprises serially connected from ground: aresistor 6 of value r through which current I of magnetron 1 passes, thepositive terminal of source 2 being connected to the top terminal of 6,a source 7 of adjustable DC low-voltage V, which is assumed to be madeof a third AC current rectifier and of filter circuits not shown, asource 8 of c.e.m.f. e, which is very stable and has a very low internalresistance, a resistor 9 of value r,, and the input resistance r, ofamplifier 5.

It will be assumed that sources 2, 4 and 7 have low internal resistancescompared to those of the circuits to which they are supplying current;and also, it will be assumed that r r, r

A simple calculation shows that input current is:

e (n I+Ve' o/ 2) 1) Considering the formula (I) of magnetroncharacteristics:

if the current gain of amplifier 5 is unity and if output resistor R of5 is very high, by combining (l) and (7) the following formula isobtained for current I:

l=[V(r, .-l-r )/r(r,+r +ar -a( V 'e,,/r -l -r -l -a 1) From (8), itappears that when there is no feedback of current I to input ofamplifier 5 (r O), the transconductance I/V, is equal to (a/r, r whilewith current feedback, it is (a/r r a 'r,)

and, thus, is reduced by the ratio (r, r lr r, a r,).

If a r, is very much higher than r r the transconductance becomesapproximately l/r, and, consequently, is independent of a; if thesources are stable, current I and microwave power P provided by themagnetron are independent of possible instabilities of static magnetronor amplifier characteristics, such as for example resulting fromtemperature variations.

From 8), microwave power delivered by the magnetron is:

For constant high-voltage V, P,,, varies within the desired range byadjusting V and/or r,.

In practice, additional sources 4 and 7 will be first turned on, beforeapplying high-voltage V to source 2. In such conditions, since there isno magnetron current feedback to the input of amplifier 5, current I incoil 3 Such an intensity is then lower then that passing through coil 3,with the same voltage V when magnetron 1 is under operation; but, in thepresent power supply, there is no risk of overcurrent when magnetronhigh-voltage is applied and whatever the value of V is at that time,since, due to negative feedback through resistor r any abnormalmagnetron current increase is immediately throttled. It is to be notedthat this result is completely independent of the presence or not of thesource 8 of c.e.m.f. c the purpose of which will now be examined.

Source 8 permits stabilization of microwave power P,,, when DC voltagesof sources 4 and 7 are subjected to relative equal variations,particularly when voltages V, V and V, are produced by rectifying asingle source of AC current after suitable filtering. For each adjustment of P and therefore, of V, and/or r as a result of combination offormulas (8) and (9), P,,, remains constant if V and V are subjected tothe same relative instabilities: d V 'lV dV/V, when 1, [r (r, r /a)] ande, are bound by the formula Thus, to adjust power P,,,, it is necessaryonly to vary the value of r in accordance with the formula 10).

It is to be noted that voltage V of source 4 as well as variationsthereof do not interfere with the problem of stabilizing P That resultsfrom the fact that dynamic output resistance R, of amplifier 5 isassumed to be high when compared to R or, in other words, amplifier 5operates as a constant current source.

It is of interest to use the power supply with values of r, such as: arr +r 4 In such conditions, the formula 10) becomes:

2 I r e,, and electrical power applied to the magnetron is stated in avery simple manner by the formula:

P= VI= Ve /2r (withP =k(Ve /ar,), (11) P and P being no longer dependingon stable elements (c and r From a combination of (8) and (10), stillassuming that a r r, r the value of voltage V. of source 7 issubstantially constant and equal to:

Considering again the formula (1), War measures the coil current (1 thatprecludes magnetron 1 from operating when voltage of source 2 is equalto V; thus The amplifier may include a suitable silicon or germaniumpower transistor of an NPN type.

The c.e.m.f. e of the source 8 may be provided by a Zener diode.

An example using practical figures will make the device more readilyunderstandable. The following gives various values for a magnetron powersupply operating in continuous mode and able to provide, in addition tothe maximum microwave power (P any value between (P,,.),, and A(P,,,),,, each value being adjusted and regulated according to thisinvention.

The magnetron high-voltage is 5 kV and current intensity providingmaximum power (P is 1 A. With an efiiciency ratio 1: of percent, (P,,,)3.5 kW. The four power levels are obtained for current values 1, 0.75,0.5 and 0.25 A. The value of (1 is 1.3 A and magnetron figure a iswithin the range of 35; in other words, current 1,, corresponding tocurrent I equal to 1 A is 1.27 A. Amplifier current gain is about 30.Powertransistor emitter-base resistance r for a base current of 40 mA islower than 15 ohms. Since this resistance varies in response totemperature changes, it is useful to mask it in the sum (r,, r by havingthe value of r substantially higher, i.e. ohms.

If the current I is of 1 A and value of r, of 1.5 ohms, condition a r, rr is amplyfulfilled. It is still better fulfilled when r is increased toprovide lower currents I.

The formula (10) gives e 3 V (a value that is readily provided by aZener diode) and the formula (12) gives: V,,' 1.5 1.3(/30) 6.5 V.

Resistor 6, which in this case must continuously vary from 1.5 to 6ohms, is made of a rheostat.

FIG. shows a detailed circuit embodiment of the invention using acommon-emitter power-transistor amplifier of the NPN type having thesame reference nu meral 5 as in the FIG. 1. Other like referencenumerals will be used for the same components already shown in FIG. 1.

Rectifier 10 followed by filter stage 11 provides voltage V that isapplied through resistor R, of coil 3 to the collector of transistor 5.

Voltage V is produced by means of a voltage divider 12, having arelatively low resistance, serially connected with resistor of rheostat6, to provide resistor values r suitable for current feedback andstabilization according to the invention. Voltage V, from slide contactof 12 is applied through Zener diode 8 and resistor 9 to the base oftransistor 5. Capacitor 13, connected between the transistor base andground, removes AC signals which remain at the output of filter 11.

Divider 12 has a division ratio that is slightly adjustable so as toadjust V at a suitable constant value,

either-at an initial adjustment or, from time to time, duringmaintenance operations. Again, it is noted that resistor r is equal tothe sum of resistor 9, the divider resistor between the top point andrheostat, and the rheostat resistor.

Power diode 16 is connected in parallel with coil 3 in a backwarddirection with respect to I Its purpose is to avoid damaging transistor5 in the case of a power supply break-down. If such an event occurs, themagnetic energy stored in coil 3 is dissipated in 16, so as to protecttransistor 5.

High-voltage V of magnetron 1 is provided by rectifier 14 followed byfilter stage 15. Rectifiers and 14 are supplied by a single primary ACsource, not shown, that, according to the invention may be anon-stabilized source.

While the principles of this invention have been described above inrelation to several embodiments, it is to be clearly understood thatthis has been only made by way of example and does not limit the scopeof the invention, as set forth in the appended claims.

What is claimed is:

l A power supply circuit for a continuous wave magnetron capable ofdelivering microwave power that is adjustable and stable for eachadjustment, comprising:

a magnetron having an anode and cathode, said anode being connected to aground reference,

a first source of constant high DC voltage applied between said anodeand cathode to provide magnetron current,

a second constant low direct voltage source,

a current amplifier and magnetron load coil serially connected to saidsecond source, the amplifier output resistance being substantiallyhigher than the coil resistance,

current feedback means connected between the input of said currentamplifier and ground and having a voltage dividing connection to saidfirst source of voltage feeding in adjustable portion of current fromsaid magnetron to said amplifier input, and

a third adjustable low direct voltage source connected between saidsecond source and said amplifier input, the amplifier input currentbeing the 3. The power supply circuit according to claim 2 wherein saidfirst, second and third source voltages are unstabilized and aresupplied by a common primary AC source, said first and second sourcesincluding respective rectifiers and filters, and a source of counterelectromotive force that is stable and has a very low internalresistance connected in series with said third low voltage source.

4. The power supply circuit according to claim 3 wherein said amplifieris a common-emitter NPN type power transistor, said voltage of saidthird source being produced by an adjustable voltage dividingconnectionfrom the voltage of said second source, said source of counterelectromotive force includes a Zener diode,

said adjustable resistor being an adjustable potentlometer-connected inseries between said third voltage source and ground, and thepotentiometer slide contact cluding a power diodeconnected in parallelwith said coil, the backward diode direction being that of the coilcurrent.

1. A power supply circuit for a continuous wave magnetron capable ofdelivering microwave power that is adjustable and stable for eachadjustment, comprising: a magnetron having an anode and cathode, saidanode being connected to a ground reference, a first source of constanthigh DC voltage applied between said anode and cathode to providemagnetron current, a second constant low direct voltage source, acurrent amplifier and magnetron load coil serially connected to saidsecond source, the amplifier output resistance being substantiallyhigher than the coil resistance, current feedback means connectedbetween the input of said current amplifier and ground and having avoltage dividing connection to said first source of voltage feeding inadjustable portion of current from said magnetron to said amplifierinput, and a third adjustable low direct voltage source connectedbetween said second source and said amplifier input, the amplifier inputcurrent being the sum of current from said third source and saidadjustable current portion.
 2. The power supply circuit according toclaim 1 including a resistor connected between the amplifier input andsaid third source, said voltage dividing connection including anadjustable resistor in series with said third low voltage source betweenthe positive terminal of said first high voltage source and ground, saidadjustable resistor being selected so that the negative feedbackresistance is very much higher than the sum of the amplifier inputresistance and said resistor from said amplifier input to the thirdsource.
 3. The power supply circuit according to claim 2 wherein saidfirst, second and third source voltages are unstabilized and aresupplied by a common primary AC source, said first and second sourcesincluding respective rectifiers and filters, and a source of counterelectromotive force that is stable and has a very low internalresistance connected in series with said third low voltage source. 4.The power supply circuit according to claim 3 wherein said amplifier isa common-emitter NPN type power transistor, said voltage of said thirdsource being produced by an adjustable voltage dividing connection fromthe voltage of said second source, said source of counter electromotiveforce includes a Zener diode, said adjustable resistor being anadjustable potentiometer connected in series between said third voltagesource and ground, and the potentiometer slide contact being connectedto the positive terminal of said first source.
 5. The power supplycircuit according to claim 4 including a high value capacitor connectedbetween said transistor base and ground.
 6. The power supply circuitaccording to claim 4 including a power diode connected in parallel withsaid coil, the backward diode direction being that of the coil current.